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A classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics.

The most popular stellar classification system on Terra, thus becoming Earth Starfleet standard (2065 to 2161 CE) and Federation standard (since 2161 CE) later, is described below. It was developed by Annie Jump Cannon of the Harvard College Observatory from 1915 to 1924 CE while she and her colleagues were ordering 225,300 stars into stellar spectra of types O, B, A, F, G, K and M into what was published as the Henry Draper Catalogue shortly after. This classification inspired the mnemonic phrase "Oh Be A Fine Girl/Guy—Kiss Me (Right <Smack> Now)!" still taught to astronomy students today.

The Hertzsprung-Russell diagram (HRD) relates stellar classification with absolute magnitude, luminosity, and photospheric temperature.

Class Characteristics Color Photospheric Temperature (K) Examples
O Ionized helium (He II) blue 28000–50000 Mintaka
B Neutral helium (He I), Balmer series hydrogen bluish white 9900–28000 Rigel, Spica
A Hydrogen, calcium (Ca II) white 7400–9900 [[acamar]], Sirius, Vega
F Calcium (Ca II), metals yellowish white 6000–7400 Canopus, Procyon
G Calcium (Ca II), iron and other metals yellow 4900–6000 Capella, Sol (G2)
K Large share of metals, later titanium oxide orange 3500–4900 Arcturus, Aldebaran
M Titanium oxide reddish orange 2000–3500 Antares, Betelgeuse, Kapteyn's Star
R Cyan (CN), carbon monoxide (CO), carbon reddish orange 3500–5400
S Zinc oxide reddish orange 2000–3500
N Carbon red 1900–3500